Method of providing alloy contacts on semi-conductor bodies



I Jan. 25, 1966 ool T 3,230,609

METHOD OF PROVIDING'ALLOY CONTACTS ON SEMI-CONDUCTOR BODIES Filed Jan. 50, 1962 FIG.3

INVENTOR ELSE KODI ALBERT SCH nz United States Patent 9 Claims. ((51. 29-1555 The invention relates to a method of providing an alloy contact on a semi-conductor body, in which an amount of an alloying material is disposed on the semi-conductor body and fused and/ or alloyed thereto at a temperature exceeding 500 C. with the use of a flux. Alloy contacts on semi-conductor bodies are used, for example, in semiconductive electrode systems, such as transistors and diodes.

According to known methods of the above-mentioned kind, comparatively readily evaporating fluxes, such as ammonium fluoride or zinc chloride, were used in the area in which the alloying material and the semi-conductor body were brought into contact. The flux was applied, for example, to a pellet of the alloying material or to the part of the surface of the semi-conductor body where the alloy contact 'Was to be provided. However, at temperatures exceeding 500 C. these fluxes rapidly disappear either due to the high vapour pressure of the flux itself or by the formation of volatile decomposition products, while furthermore in some cases a comparatively large amount of residue may be left by which the contact is contaminated.

It has also been proposed to use hydrochloric acid as a flux, an aqueous solution of hydrochloric acid being provided separately from the alloying material and being heated in the alloying process. However, the hydrochloric acid readily evaporates so that it has completely disappeared at a temperature of 500 C. and hence no longer acts as a flux. The surface of the semi-conductor body and the alloying material may, however, be highly contaminated previously by the action of the vapour.

It is an object of the invention to avoid the abovementioned disadvantages. According to the invention, the flux contains at least one alkali metal halide and it is provided separate from the alloying material and is also heated in the fusing and/or alloying process. Alkali metal halides generally are only slightly volatile at temperatures below 500 C. and gradually vaporize at temperatures above 500 C. so as to be capable of providing an even supply of a vapour of not excessive pressure promoting fusion to the alloying material and the surface of the semi-conductor body to be wetted by this material. The flux may be positioned in close proximity of the alloying material, its temperature may then be substantially equal to the temperature of the alloying material or somewhat lower due to the absorption of evaporation heat. The flux preferably contains an alkali metal fluoride. In a further preferred embodiment of the invention, the flux contains a sodium halide. In practice, mixtures of sodium fluoride and sodium chloride, for example mixtures of from 10 to 50% by weight of NaF, the remainder being NaCl, have proved highly suitable.

Also sodium bromide and sodium iodide may be used in the method, solely or mixed together or mixed with sodium chloride and/or sodium fluoride. The iodide and bromide are suitable especially in cases of fluxing at temperatures above 500 C. and under 900 0., being somewhat more volatile than the chloride and fluoride. Instead or in addition to sodium halogenides, potassium halogenides solely or mixed are also suitable. The same applies for rubidium and caesium halogenides, which, however, are more expensive.

3,230,609 Patented Jan. 25, 1966 The method is particularly suited to alloying and/ fusing alloying materials containing aluminum, inclusive of aluminum itself. As is known, the adherence of these materials is impeded by the presence of aluminum oxide. When the above-mentioned known halogen containing fluxes are used, their intense action may cause a considerable part of the aluminum of the alloying material to be converted into volatile halides so that it no longer takes part in the alloying process. In the method according to the invention, no inconvenient decrease of the aluminum content of the contact due to the formation of aluminum halides occurs.

Although the method according to the invention may be used in alloying contacts to many semi-conductor materials, for example germanium, it has proved particularly suited to the manufacture of alloy contacts on silicon bodies, specially in the processes of fusing or alloying at a temperature of at least 900 C. Such high temperatures are used, for example, when applying alloying materials consisting at least substantially of tin.

Other combinations of alloying materials and semi-conductors with which the method may be carried out successfully are for instance alloying materials consisting substantially of lead, with germanium, and alloying materials consisting mainly of indium, with silicon. The invention is not limited to the above materials and the method may be used in many other cases, especially those in which suitable wetting of the surface of the semi-conductor body by the electrode material will occur at temperatures above 500 C.

In order that the invention may readily be carried into effect, two embodiments thereof will now be described, by way of example, with reference to the accompanying drawings which are diagrammatic vertical sectional views of stages of the provision of alloy contacts on semi-conductor bodies.

FIGURE 1 shows an alloying jig containing a semiconductor body on which is arranged a pellet of the material to be alloyed thereto.

FIGURE 2 shows the same alloying jig as FIGURE 1 after being subjected to a heat treatment.

FIGURE 3 shows another jig containing a semi-conductor body on which is arranged a pellet of alloying material.

FIGURE 4 shows the same semi-conductor body as FIGURE 3 provided with an electrode alloyed thereto by heating.

Example I In this example an alloying jig 1 is used comprising a graphite body 2 and a cover plate 3 likewise made of graphiteand provided with an aperture 4 (FIGURE 1). A semi-conductor body 5 consisting of n-type silicon having a resistivity of 2 ohm-cm. is accommodated in a recess of the graphite body 2. An amount of about 1 mgm. of a powder 7 consisting of a eutectic mixture of 27.5 parts by weight of sodium fluoride and 72.5 par-ts by weightof sodium chloride is arranged in a cup-shaped recess 6 in the body 2. The cover plate 3 is then laid on the body 2 and subsequently a pellet 8 of aluminum is placed in the aperture 4 on the silicon body 5.

The filled jig is inserted in a vitreous quartz tube 9. A stream of hydrogen is passed through this tube and the tube is put in a furnace, for example a tubular furnace, which is not shown. The alloying jig 1 is then heated to about 750 C. during about five minutes so that the pellet 8 melts. The mixture 7 then also melts and slowly vaporizes. The resulting vapour acts upon the molten aluminum and upon the surface of the silicon body 5 so that this surface is satisfactorily wetted by the molten aluminum. The aluminum is allowed with the subjacent silicon and the molten alloy penetrates-into the silicon body with a substantially flat front.

After cooling, an alloy contact 10 with a substantially flat p-n junction 11 is obtained (FIGURE 2).

Example II A semi-conductor body 21 of n type silicon having a resistivity of 1 ohm-cm. is arranged in a recess of a graphite jig 20 (FIGURE 3). A pellet 22 consisting of a tin alloy containing 0.5% by weight of aluminum is glued to the silicon body by means of an adhesive.

A small amount of powdered sodium fluoride 24 is arranged in a cup-shaped recess 23. The filled jig is inserted into a vitreous quartz tube 25 and a stream of pure hydrogen is passed through the tube. The jig 20 is now heated to 1150 C. by means of a furnace (not shown) for 3 minutes. The pellet melts and the adhesive by which the pellet was made to adhere to the silicon disappears. The sodium fluoride in the recess 23 gradually vaporizes and the vapour acts upon the surface of the silicon body 21 and upon the molten tin aluminum alloy of the pellet 22, any inconvenient oxide film being removed, while the molten material satisfactorily wets the surface of the silicon body. The front between the melt and the solid material of the semi-conductor body gradually penetrates into the semi-conductor body, a slight amount of silicon dissolving in the melt. From the front aluminum diffuses into the solid material of the body with the formation of a gradual p-n-junction.

After cooling, a rectifying alloy diffusion contact 26 is formed on the n-type silicon body 21 (FIGURE 4).

The alloy contacts produced on semi-conductor bodies by the method described in the above examples may be used in semi-conductive electrode system. On the side of the semi-conductor body or 21 respectively opposite to the side on which the rectifying alloying contact or 26 respectively is provided, an ohmic contact may be provided on the n-type silicon of the body by means of a gold-antimony or gold-tin-antimony alloy to produce a diode.

Obviously, a number of alloy contacts may be provided on a semi-conductor body by the method according to the invention. Furthermore, the invention is not restricted to making rectifying contacts but it also includes the provision of other alloy contacts, for example ohmic contacts, on semi-conductor bodies.

What is claimed is:

1. A method of making a semi-conductor device by alloying, comprising the steps of providing a semi-conductive body and a mass of alloying electrode material, providing in the vicinity of the electrode mass and semiconductive body but separate therefrom a suply of a flux containing at least an alkali metal halide, heating the semiconductive body and electrode mass at a temperature exceeding 500 C. to melt the mass and contacting the semiconductive body with the melted mass while simultaneously heating the flux supply to vaporize the alkali metal halide and provide vapors thereof at the contacted mass and body to promote melting and fusion and allowing thereof, said alkali metal halide being substantially involatile at temperatures below 500 C. and remaining stable at the elevated temperature at which alloying occurs, and cooling the assembly to form an electrode mass alloyed to the semiconductive body.

2. A method as set forth in claim 1 wherein the flux comprises an alkali metal fluoride.

3. A method as set forth in claim 1 wherein the flux comprises a sodium halide.

4. A method as set forth in claim 1 wherein the flux comprises essentially a mixture of sodium fluoride and sodium chloride.

5. A method of making a silicon semi-conductor device by alloying, comprising the steps of providing a silicon semiconductive body and an aluminum-containing mass of alloying electrode material, providing in the vicinity of the electrode mass and silicon body but entirely separate therefrom a supply of a flux containing at least an alkali metal halide, heating the semiconductive body and electrode mass at a temperature exceeding 500 C. to melt the mass and contacting the semiconductive body with the melted mass while simultaneously heating the flux at substantially the same temperature to vaporize the alkali metal halide and provide solely vapors thereof at the contacted mass and body to promote melting and fusion and alloying thereof, said alkali metal halide being substantially involatile at temperatures below 500 C. and remaining stable at the elevated temperature at which alloying occurs, and cooling the assembly to form an electrode mass alloyed to the semiconductive body.

6. A method as set forth in claim 5 wherein the flux is selected from the group consisting of sodium fluoride and sodium chloride.

7. A method of making a silicon semi-conductor device by alloying, comprising the steps of providing a silicon semiconductive body and a mass of alloying electrode material, providing in the vicinity of the electrode mass and silicon body but separate therefrom a flux containing at least an alkali metal halide, heating the semiconductive body and electrode mass at a temperature exceeding 900 C. to melt the mass and contacting the semiconductive body with the melted mass while heating the flux to vaporize the alkali metal halide and provide solely vapors thereof at the contacted mass and body to promote melting and fusion and alloying thereof, said alkali metal halide being substantially involatile at temperatures below 500 C. and remaining stable at the elevated temperature at which alloying occurs, and cooling the assembly to form an electrode mass alloyed to the semiconductive body.

8. A method as set forth in claim 7 wherein the mass is comprised principally of tin.

9. A method as set forth in claim 7 wherein the flux isselected from the group consisting of sodium fluoride and sodium chloride.

References Cited by the Examiner UNITED STATES PATENTS 2,299,166 10/ 1942 Miller 29495 X 2,561,565 7/1951 Edson et a1. 29494 2,674,790 4/1954 Edson et a1 29495 X 2,800,711 7/1957 Oliphant 29-495 X 2,877,147 3/1950 Thurmond l481.5 2,996,800 8/1961 Holly 29495 X 3,002,864 10/1961 Van Amstel 29495 X 3,015,591 1/1962 Zaratkiewicz et al. 1481.5 3,043,726 7/1962 Jochems 148-1.5

JOHN E. CAMPBELL, Primary Examiner. 

1. A METHOD OF MAKING A SEMI-CONDUCTOR DEVICE BY ALLOYING, COMPRISING THE STEPS OF PROVIDING A SEMI-CONDUCTIVE BODY AND A MASS OF ALLOYING ELECTRODE MATERIAL, PROVIDING IN THE VICINITY OF THE ELECTRODE MASS AND SEMICONDUCTIVE BODY BUT SEPARATE THEREFROM A SUPLY OF A FLUX CONTAINING AT LEAST AN ALKALI METAL HALIDE, HEATING THE SEMICONDUCTIVE BODY AND ELECTRODE MASS AT A TEMPERATURE EXCEEDING 500*C. TO MELT THE MASS AND CONTACTING THE SEMICONDUCTIVE BODY WITH THE MELTED MASS WHILE SIMULTANEOUSLY HEATING THE FLUX SUPPLY TO VAPORIZE THE ALKALI METAL HALIDE AND PROVIDE VAPORS THEREOF AT THE CONTACTED MASS AND BODY TO PROMOTE MELTING AND FUSION AND ALLOWING THEREOF, SAID ALKALI METAL HALIDE BEING SUBSTANTIALLY INVOLATILE AT TEMPERATURES BELOW 500*C. AND REMAINING STABLE AT THE ELEVATED TEMPERATURE AT WHICH ALLOYING OCCURS, AND COOLING THE ASSEMBLY TO FORM AN ELECTRODE MASS ALLOYED TO THE SEMICONDUCTIVE BODY. 